Present refrigeration systems, including those for cryogenic applications, are almost entirely based on successive compression and expansion cycles of a gas. Generally, the efficiency of practical gas cycle refrigerators is only a fraction of the ideal Carnot cycle efficiency, and the efficiency generally decreases with a decrease in the size of the refrigerator. The efficiency of gas cycle refrigerators is particularly low at cryogenic temperatures, e.g., in the 2 Kelvin (K) to 20K range.
It has long been known that certain magnetic materials exhibit the magnetocaloric effect: they increase in temperature when placed in a magnetic field and decrease in temperature when removed from the field. Application of a magnetic field to such solid magnetic materials is analogous to compressing a gas (producing an increase in temperature), and removing the field from the solid is analogous to expanding a gas (producing a decrease in temperature). Thus, it has been recognized that a thermodynamic refrigeration cycle can be achieved using a magnetic material as the working material in a manner analogous to the refrigeration cycles of a gas. Examples of relatively recent designs proposed for magnetic refrigerators are shown in U.S. Pat. Nos. 4,033,734, 4,069,028, 4,107,935, 4,332,135, 4,392,356, 4,408,463, 4,441,325, 4,457,135, 4,459,811, 4,464,903, 4,507,927, and 4,507,928.
As a general rule, the effective temperature range of the magnetocaloric effect for any magnetic solid is considerably more limited than the working temperature range for gases. Typically, a magnetic refrigerator using a single magnetic material has a characteristic useful temperature range on each side of its magnetic ordering temperature T.sub.0, beyond which the material becomes either magnetically saturated or weakly magnetic. However, large temperature differentials between the ambient and the cooled medium can be obtained by utilizing cascaded magnetic refrigerators, each using a material with an appropriate magnetic ordering temperature.
The magnitude of the magnetocaloric effect in a given material depends directly on the magnitude of the magnetic field applied to the material. To take full advantage of the magnetocaloric effect requires very high magnetic fields, preferably several Tesla, thus usually dictating that superconducting magnets be utilized rather than less efficient normal conducting magnets. Prior designs for magnetic refrigerators have typically required liquid helium baths to cool the superconducting magnet windings below the critical temperature of the superconductor used. Thus, such refrigerators require initial priming with liquid helium before they can be operated. In addition to the need to provide liquid helium to the refrigerators for cooling of the superconducting coils, the heat transfer mechanisms of most proposed magnetic refrigerators have required the pumping of a gas or liquid to achieve heat transfer. As an alternative to active heat transfer in magnetic refrigerators by a gas or liquid, it has been proposed to use heat pipes which transport heat by an evaporation-condensation cycle of a fluid--helium or hydrogen for refrigerators operating below 20K.
The need to have liquid helium available before cryogenic magnetic refrigerators can be started up can be a significant disadvantage, particularly where the magnetic refrigerator is designed to operate as a stand-alone device away from other cryogenic refrigeration equipment which could be a source of liquid helium, for example in space flight applications. The ability to start up without liquid helium is also desirable for equipment which may be used intermittently or periodically, such as medical equipment which requires cooling of superconducting magnets, and where it is inconvenient or expensive to obtain and store liquified helium in preparation for the start-up of the machine.
The use of high field superconducting magnets to produce the most efficient magnetic refrigeration imposes substantial mechanical loads on the support structure of the refrigerator. The support structure must be sufficiently massive and rigid to resist the forces imposed without substantial deformation, and yet must not constrain the mechanical operation of the magnetic refrigerator or create a significant thermal addenda with consequent loss of refrigeration efficiency, and must not permit undue heat transfer between hot and cold regions.